1. Field of the Invention
The present invention relates to a hydraulic brake apparatus for a vehicle, and more particularly to the apparatus having an auxiliary pressure source for generating a power pressure and a valve assembly for regulating the power pressure into a regulated pressure.
2. Description of the Related Arts
As for the hydraulic brake apparatus having the auxiliary pressure source and the valve assembly, the U.S. Pat. No. 3,928,970 discloses power-assisted master cylinder assemblies which comprise a piston movable in a cylinder bore, a pressure space ahead of the piston having an outlet for connection to a brake circuit, a power chamber behind the piston having an inlet for connection to a source of fluid under pressure, and a control valve assembly for controlling the supply of fluid from the pressure source to the power chamber. It is an object of the invention disclosed in the above United States Patent to provide a power-assisted master cylinder assembly in which the piston is pedal operated but which does not require a movable inlet or inlet seals which move under pressure. According to the assembly as disclosed above, the power-assisted master cylinder includes a pedal operated first piston for pressurizing the pressure space connected to the brake circuit, and a second piston for operating valve means controlling the fluid assisting the pedal, movement of the first piston being transmitted to the second piston either hydraulically or mechanically. In that assembly, one end of the second piston is exposed to the pressure in the pressure space and the other end is exposed to the pressure of the assisting fluid.
According to the assembly as disclosed in the above United States Patent, when the brake pressure in the wheel cylinder is to be controlled in response to depression of the brake pedal, the control valve assembly (or valve assembly) has to be operated rapidly in response to a small variation of depressing force applied to the brake pedal, in fact a variation of master cylinder pressure caused by the variation of depressing force. In the assembly as disclosed in the above United States Patent, however, the control piston is provided with two seals, each seal having one end exposed to the pressure in the pressure space or the pressure of the assisting fluid, and the other end exposed to the pressure in the space leading to the reservoir under the atmospheric pressure. Therefore, a large pressure difference is applied to the seals, thereby to deform the seals. As a result, the frictional force of the seals will be increased to cause an excessive hysteresis in an input-output characteristic of the brake pressure, for example, so that it will be difficult to achieve a desired power-assisted operation.